The present invention relates to improvements in or relating to ovens intented for drying paints on continuously travelling metal bands, strips or tapes.
It is known that the drying of paints used in such processes is attended, according to the specific nature of the paints, by the release of water steam or organic solvent vapours. In this last instance, highly combustible products are involved which must be diluted sufficiently to prevent their ignition within the oven enclosure. This dilution is obtained by using air in such proportions as to set the mixture well beyond the inflammability limits.
Therefore, in hitherto known paint drying ovens one portion of the gases contained in the oven is discharged permanently and the organic solvents are burned in an incineration oven meeting a twofold requirement: on the one hand, eliminating any possibility of pollution by these organic substances, and on the other hand regenerating the heat contained therein in order to improve the energy balance of the drying plant.
Nowadays, ovens of relatively large size, comprising very complicated gas circulation systems, are used for the continuous drying of paints deposited on bands or strips. A typical example of an oven of this type is illustrated diagrammatically in FIG. 1 of the attached drawing.
In FIG. 1 the reference numeral 10 designates the drying oven proper. This oven is divided into several sections A, B, C and D in order to obtain different heating rates for each phase of the paint drying cycle. In fact, at the beginning of this cycle the primary effect is to substantially evaporate the organic solvents contained in the paints. Then, the paints are polymerized, and finally hardened. As a result, very variable solvent concentrations are recorded along the path followed by the band travelling continuously through the oven. As clearly shown in the drawing, each section of the oven illustrated comprises a gas blowing device 12 so that the gas impinges against the continuously travelling band 14 and adequate heat exchanges take place by convection. The gas thus blown against the band 14 is taken from the surrounding gas in chamber 10' of the oven and includes fresh air mixed with hot gases (circulating along the circuit shown in dash lines) from the incinerator 16 and from the smokes produced in a make-up burner such as 18 for keeping the mixture temperature at a value substantially within the range of 150.degree. to 450.degree. C.
At a predetermined point P of the evaporation area of oven 10 a certain output of the gaseous mixture is removed from the oven, the solvent content of this mixture resulting from the solvent evaporation taking place during and along the drying cycle. This mixture circulating along the path shown in dash and dot lines is directed to the incineration oven 16 in which the solvents are burned by virtue of the additional heat resulting from a preferably gaseous make-up fuel G.
In this known arrangement, the gas temperature at the outlet of the incineration oven is of the order of 750.degree. C. These gases are directed on the one hand towards the oven sections A, B, C, and D (along the dash-line circuit) in which they are mixed as mentioned hereinabove with the surrounding gas before being blown against the metal band, and on the other hand towards heat regenerating means 20, for example in order to produce steam or hot water, for heating thermal fluids, etc.., before being discharged to the atomsphere via a conduit 22. The fraction of the gases thus rejected to the atmosphere is compensated by the ingress of fresh air into the oven, such as shown at the right side thereof in FIG. 1.
In a modified construction, not shown in the drawing, of this type of known arrangement, the gases recycled to the various sections of the drying oven are derived not directly from the incinerator 16 but consist of air heated by means of a heater interposed between the incinerator 16 and the heat regenerating means 20.
The capacities of incinerators of the type now widely used range from about 200 to about 500 liters of solvent per hour. However, it is contemplated for future plants to improve these capacities up to and even beyond 1,000 liters/hour with a corresponding air output of the order of 70,000 Nm.sup.3 /h. Consequently, the incineration ovens incorporated in plants of this type will be extremely bulky. Besides, the pipes connecting the incineration oven to the various sections of the drying oven will convey gases at temperatures of the order of 750.degree. C., so that efficient lagging must be used, which means an excessive increase in costs and dimensions.
On the other hand, due to the considerable mass of solvents to be evaporated, it is very important to provide means for equalizing their concentration along the oven in order to minimize the dilution outputs necessary for keeping the mixtures below their inflammability limits.
Moreover, these known plants are also objectionable in that the drying oven exploitation is subordinate to the incineration requirements, on the one hand, and to the energy recovery or recuperation, before rejecting the gases to the atmosphere, on the other hand.